1. Field of the Invention
The present invention relates to a tyre for vehicle wheels, to a tread band and to a crosslinkable elastomeric composition.
More particularly, the present invention relates to a tyre for vehicle wheels, comprising at least one component made of a crosslinked elastomeric material including at least one salt or one oxide of a transition metal belonging to groups IIIA to VIIIA of the Periodic Table of the Elements.
The present invention moreover relates to a tread band including a crosslinkable elastomeric composition comprising at least one salt or one oxide of a transition metal belonging to groups IIIA to VIIIA of the Periodic Table of the Elements, and also to an elastomeric composition comprising at least one salt or one oxide of a transition metal belonging to groups IIIA to VIIIA of the Periodic Table of the Elements.
2. Description of the Related Art
Processes for vulcanizing diene elastomeric polymers with sulphur are widely used in the rubber industry for the production of a wide range of manufactured products, and in particular of tyres for vehicle wheels. Although these processes lead to the production of high-quality vulcanized products, they include considerable complexity mainly linked to the fact that, in order to obtain an optimum vulcanization in industrially acceptable times, it is necessary to use a complex vulcanizing system which includes, in addition to sulphur, one or more activators (for example, zinc compounds, and in particular ZnO, ZnCO3, zinc salts of fatty acids such as, for example, zinc stearate) and one or more accelerators (for example, thiazoles, dithiocarbamates, thiurams, guanidine, sulphenamides). The presence of these products may, in some cases, entail considerable problems as regards the harmfulness/toxicity both when being produced and when in use, in particular when the vulcanized manufactured products are intended for medical-health or food use.
The zinc compounds described above are counted among the products that give rise to the abovementioned drawbacks. In this respect, a number of attempts have been made directed towards reducing or eliminating the amount of said compounds, in particular of zinc oxide.
Patent application WO 00/37267 in the name of the Applicant discloses a crosslinkable elastomeric composition that is particularly useful for producing tyre tread bands, comprising: (a) a polymer base containing a crosslinkable unsaturated chain; (b) a vulcanizing system including: (b1) an amount of between 0.5 phr and 2 phr of sulphur, (b2) an amount of between 1.5 phr and 7 phr of at least one vulcanization accelerator containing at least one carbon atom linked to at least two sulphur atoms, (b3) an amount of not greater than 2 phr, expressed in terms of zinc oxide equivalents, of at least one activator. Although reducing the amount of activator, it would thus be possible to obtain a tyre with improved wear resistance capable of maintaining unchanged its characteristic properties of wet road holding and of rolling resistance.
“International Polymer Science and Technology” (1994), Vol. 21, No. 7, pp. 48-51 describes the possibility of reducing the amount of zinc oxide in crosslinkable elastomeric compositions. To this end, a complex which consists of zinc oxide/sulphur/stearic acid/2-mercaptobenzo-thiazole/tetramethylthiuram disulphide is added to said compositions instead of the mechanical mixture of said products that is normally used. In this way, an increase in the rate of vulcanization and a reduction in the vulcanization induction time at low temperatures are said to be achieved.
U.S. Pat. No. 3,856,729 describes a crosslinkable elastomeric composition comprising a butadiene rubber, sulphur and an activator, characterized in that the sulphur and the activator are present in an amount of between 0.1 phr and 0.8 phr, preferably between 0.1 phr and 0.6 phr, and between 0.1 phr and 1.0 phr, preferably between 0.1 phr and 0.5 phr, respectively. Said amounts are appreciably lower than the amounts normally used. The crosslinked manufactured product obtained is said to have good tear strength, good resistance to elongation and good resistance to ageing.
U.S. Pat. No. 3,451,458 describes a sulphur-crosslinkable elastomeric composition that is particularly useful for preparing tyre tread bands, comprising a synthetic diene rubber and silica, said composition being substantially free of activators based on metal oxides, and in particular zinc oxide. The absence of zinc oxide is said to make it possible thereby to obtain tyres with good tensile strength and good wear resistance.